In High Frequency (HF) communications, the carrier is normally not transmitted with the signal. Such a suppressed carrier signal is more efficient to transmit, since the signal bandwidth is reduced. However, carrier suppression makes it more difficult for the receiver to properly demodulate the signal. In tuning a HF receiver, the process is the same as used to tune an AM radio dial to an AM station. The primary difference is that AM radio station's broadcast on a precisely known, assigned carrier frequency.
By contrast, HF radios do not operate on assigned frequencies. Operators must either agree on a carrier frequency before transmission, or the operator must manually tune the receiver until the signal “sounds right.”
U.S. Pat. No. 5,684,842, entitled “DIGITAL TRANSMISSION SYSTEM COMPRISING A RECEIVER WHICH INCLUDES A CARRIER RECOVERY CIRCUIT,” discloses a method removing phase jitter from a carrier signal. The correction is estimated using the known carrier frequency, or one that has been encoded by an agreed upon constellation scheme, and the correction is based on the phase difference of the received signal. The expected carrier signal is transmitted along with the signal or agreed to in advance. The present invention is not limited in this regard. U.S. Pat. No. 5,684,842 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,982,809, entitled “METHOD AND APPARATUS FOR CARRIER RECOVERY AND COMPENSATION IN SPREAD SPECTRUM COMMUNICATIONS,” discloses a method estimating the carrier frequency in spread spectrum communications. The spread-spectrum communications signal is demodulated to remove the expected carrier frequency, and a phase change of demodulated signal is calculated. The average phase change is used to estimate the carrier offset. In spread-spectrum communications, an expected carrier frequency is transmitted along with the signal. The present invention is not limited in this regard. U.S. Pat. No. 5,982,809 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 6,091,786, entitled “METHOD AND APPARATUS IN A COMMUNICATION RECEIVER FOR ESTIMATING SYMBOL TIMING AND CARRIER FREQUENCY,” discloses a method estimating the carrier frequency using a rough estimation from symbol timing. The selected carrier frequency is estimated by using predetermined variations of the carrier frequency about a nominal carrier frequency as a function of predetermined variations in symbol timing about the rough symbol timing. U.S. Pat. No. 6,091,786 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 6,577,968, entitled “METHOD OF ESTIMATING SIGNAL FREQUENCY,” discloses a method of estimating signal frequency by forming a cross spectrum matrix of the received signal, and selecting the magnitude in the matrix that is above a pre-determined threshold. The cross spectrum matrix is determined by taking the received signal, forming a row vector, converting the row vector into a first matrix, multiplying the first vector by a weight, taking a Fourier transform, and calculating a complex conjugate. The angular frequency of the signal is set to either the phase of the selected magnitude, the phase of the mean of the complex numbers in the row where the selected magnitude appears, or the selected magnitude. The frequency of the signal is set to the estimated angular frequency divided by the product of 2π and the signal delay period. U.S. Pat. No. 6,577,968 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 6,771,937, entitled “FREQUENCY-STABILIZED TRANSMITTING/RECEIVING CONFIGURATION,” discloses a device for correcting frequency of a received signal, especially when the received signal is moving towards or away from the receiver. The device assumes the only signal mismatch is due to the Doppler frequency shift. A known carrier frequency is assumed, and only the correction due to Doppler shifts need to be calculated. The present invention is not directed to the case where the carrier frequency is known and only the Doppler shift needs to be calculated and applied to the received signal. U.S. Pat. No. 6,771,937 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 6,839,388, entitled “SYSTEM AND METHOD FOR PROVIDING FREQUENCY DOMAIN SYNCHRONIZATION FOR SIGNAL CARRIER SIGNALS,” discloses a method of fine-tuning a carrier frequency. The method assumes a course carrier frequency has already been determined. The present invention is not lacking in this regard. U.S. Pat. No. 6,839,388 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 6,882,691, entitled “FINE-FREQUENCY OFFSET ESTIMATION,” discloses a method of fine-tuning a carrier frequency. The method assumes a course carrier frequency has already been determined. The present invention is not lacking in this regard. U.S. Pat. No. 6,882,691 is hereby incorporated by reference into the specification of the present invention.
U.S. patent application Ser. No. 11/151,173 entitled “METHOD OF REMOVING NOISE AND INTERFERENCE FROM SIGNAL USING PEAK PICKING” discloses a method of removing noise and interference from a signal. The method calculates a short time Fourier transform, and estimates a noise free and interference free signal by integration. U.S. patent application Ser. No. 11/151,173 is hereby incorporated by reference into the specification of the present invention.
Other known patents include:
U.S. Pat. No. 5,732,113 to Schmidl et al.;
U.S. Pat. No. 6,031,884 to Pon; and
U.S. Pat. No. 6,181,714 to Isaksson et al.; and
U.S. Pat. No. 6,484,112 to Scheppach.
Known non-patent publications include:
Analysis of Time-varying Signals with Small BT Values, K. Kodera, R. Gendrin, and C. de Villedary, IEEE Trans. Acoust. Speech and Signal Processing, vol. 26, pp. 64-76, 1978;
Statistically/Computationally Efficient Frequency Estimation, S. M. Kay, Proceedings of the IEEE Conference on Acoustics, Speech and Signal Processing, pp. 2292-2295, 1988;
Special Purpose Correlation Functions for Improved Signal Detection; and Parameter Estimation, Douglas Nelson, Proceedings of the IEEE Conference on Acoustics, Speech, and Signal Processing, Minneapolis, pp. 73-76, April, 1993;
Improving the Readability of Time-Frequency and Time-Scale Representations by the Reassignment Method, F. Auger and P. Flandrin, IEEE Trans. Sig. Proc., vol. 43, no. 5, pp. 1069-1089, May, 1995;
Pitch Based Methods for Speech Detection and Automatic Frequency Recovery, Douglas Nelson and Joseph Pencak, SPIE Proc Adv. Sig. Proc. Algorithms, vol. 2563, pp. 92-100, San Diego, 10-12 July, 1995;
Cross Spectral Methods for Processing Speech, Douglas Nelson, Journal of the Acoustic Society of America, vol. 110, no. 5, pt. 1, pp. 2575-2592, November 2001;
Cross-Spectral Based Formant Estimation and Alignment, Douglas Nelson, IEEE Proc. ICASSP, Montreal, April, 2004; and
Linear Distributions of Signals, Douglas Nelson and D. C. Smith, SPIE Proc Adv. Sig. Proc. Algorithms, vol. 2563, Denver, 2004.